In a printing apparatus (image forming apparatus) of electronic photography type, it is general that: an electrostatic latent image is formed by performing electrification and exposure with respect to a photoreceptor drum (image holding body), the electrostatic latent image is developed, thereby forming a toner image. After that, the toner image is transcribed to a sheet (transcription material) so as to perform printing.
As a developer used in such a printing apparatus, there are a one-component developer and a two-component developer in general classification.
The two-component developer contains not only a toner but also a carrier made of magnetic material such as Fe (iron) and ferrite, and an electrification property of the two-component developer can be adjusted by varying a mixture ratio of the carrier and the toner. Further, the two-component developer is superior in (a) a development property of a thin line and a solid image and (b) reproducibility of a gray scale, and is suitable for forming a color image.
While, the one-component developer is made only of the toner. In case of using such a developer, it is not necessary to mix and agitate the toner with the carrier, and there is such an advantage that it is not necessary to control density of the toner and it is not necessary to replace the toner with new one.
Incidentally, there is a printing apparatus of electronic photography type which performs corona discharge so as to electrify a photoreceptor drum (a member to be electrified) (for example, see Document 1: Japanese Published Unexamined Patent Application 6-50416 (Tokukohei 50416/1994, published on Jun. 29, 1994).
Note that, in the printing apparatus, a development apparatus has a function for cleaning the photoreceptor drum, thereby making the printing apparatus smaller.
However, when electrification is performed by using a corona discharger, the apparatus is susceptible to environmental influences such as humidity and dusts. Further, the foregoing arrangement brings about problems such as (a) smell caused by ozone emitted upon performing the corona discharge and (b) harmful effects exerted on a human body.
Then, as a method for avoiding such problems, the following method is known: an electrification member (conductive member), such as an electrification roller, to which a superimposing voltage has been applied, is made into contact with a surface of the photoreceptor drum so as to electrify the surface of the photoreceptor drum. Here, the superimposing voltage means a voltage prepared by superimposing an alternating current voltage on a direct current voltage.
However, such a contact electrification method brings about the following problem. That is, upon performing the printing, a relatively hard foreign material such as the carrier sometimes adheres to a surface of the photoreceptor drum or a surface of the electrification roller. At this time, in the contact electrification method, the electrification roller is in contact with the photoreceptor drum with the foreign material thereon. Thus, this condition brings about such a problem that the foreign material damages the surface of the photoreceptor drum or the electrification roller.
Then, in order to solve such problem of the contact electrification method and to achieve a no-ozone condition which is the most advantageous point, the following close electrification method has been proposed recently: the electrification roller is located close to (without touching) the photoreceptor drum.
A printing apparatus using such an electrification method is disclosed, for example, in Document 2: Japanese Published Unexamined Patent Application 2001-188403 (Tokukai 188403/2001, published on Jul. 10, 2001). The apparatus disclosed in Document 2 includes a development apparatus of two-component development type. Further, a narrowest gap (electrification gap) between a discharge surface of the electrification roller and the photoreceptor drum is set to be greater than a diameter of the carrier of the developer.
Further, in an electrification method recited in Document 3: Japanese Published Unexamined Patent Application 5-307279 (Tokukaihei 307279/1993, published on Nov. 19, 1993), the electrification gap (air gap) is set to not more than 120 μm, and a superimposing voltage (alternating voltage prepared by superimposing a low frequency alternating current voltage on a direct current voltage) is applied between the electrification roller and the photoreceptor drum.
Further, Document 4: Japanese Published Unexamined Patent Application 7-301973 (Tokukai 301973/1995, published on Nov. 14, 1995) proposes such a method that: the electrification gap (air gap) is set to 30 μm to 240 μm, and an electrode bias of a direct current component is applied to the electrification roller, so as to electrify the photoreceptor drum.
The printing apparatus of Document 2 is arranged so that the electrification gap is set to be greater than the diameter of the carrier of the developer, so that this arrangement does not bring about such a problem that the carrier and the toner adhering to the carrier are involved between the photoreceptor drum and the electrification roller. Thus, the carrier does not damage the photoreceptor drum and the electrification roller.
However, the foregoing arrangement brings about such a problem that the greater electrification gap requires a greater voltage so as to electrify the photoreceptor. Moreover, a condition under which the photoreceptor is electrified tends to be less stable as the electrification gap increases. Thus, the greater electrification gap causes lower image quality.
Further, when the electrification gap is made smaller so as to avoid such a problem (specifically, when the electrification gap is made smaller than the foreign material (such as a carrier particle of the developer), it is necessary to perform the cleaning, in upstream of the electrification roller, with respect to the photoreceptor drum, so as to prevent the photoreceptor drum and the electrification roller from being damaged or tainted (getting dirty).
Further, the foreign material (left-over material) that is on the photoreceptor drum causes deterioration of the image. A relationship between the left-over material on the photoreceptor drum and the image deterioration is described as follows.
In the printing apparatus of electronic photography type, when the toner image is transcribed onto the sheet, the toner image is not completely transcribed, so that the toner is sometimes left over on the photoreceptor drum. Further, such left-over toner causes the image deterioration such as “toner image memory” (the previous toner image remains in the following toner image). Thus, conventionally, an agitating brush for agitating the left-over toner image is used to prevent the toner image memory. However, when the agitating brush becomes dirty with the toner deposited thereon, the toner adheres to and is deposited on the electrification roller which electrifies the photoreceptor drum.
Further, when the left-over toner is deposited on the electrification roller, the electrification property of the electrification roller is deteriorated, so that this condition brings about irregular discharge. Further, the irregular discharge brings about uneven electrification of the photoreceptor drum and image fogging (black points which occur in an unexposed portion (a portion which should remain white)).
FIGS. 16(a) and 16(b) show a case where a printing apparatus which does not remove the left-over toner on the electrification roller by cleaning makes a copy of a document. Here, FIG. 16(a) is an explanatory view which shows a document image, and FIG. 16(b) is an explanatory view which shows an image generated by performing a copying operation.
As shown in these figures, when the left-over toner on the electrification roller is not removed, black points occur in a white background portion of the image, and influence exerted by the black points increases as the printing apparatus is used more frequently.
Further, FIGS. 17(a) to 17(h) are explanatory views each of which shows a condition under which the black points occur corresponding to frequency in use (the number of copied sheets) in the case where the left-over toner on the electrification roller is not removed. As shown in these figures, as the number of copied sheets increases, the black points become more influential.
Table 4 is a table which shows a relationship between the number of printed sheets (P) and a fogging value K in terms of both the case where the left-over toner on the electrification roller is removed by cleaning and the case where the left-over toner is not removed. FIG. 18 shows value of the table as a graph.
Note that, the fogging value K is calculated from an such an expression that: “K=1−U/U0, U: brightness, U0: initial brightness”.
TABLE 4Image fogging caused by the left-over toner on theelectrification rollerNUMBER OFNO CLEANINGCLEANINGSHEETS PBRIGHTNESS UFOGGING KAPPROXIMATIONBRIGHTNESS UFOGGING K 02420.000.0002420.00102400.010.0002420.00202400.010.0002420.00302330.040.0732420.00402180.100.1102430.00502110.130.1302420.00602080.140.1402420.00702070.150.1452430.00802040.160.1472430.00902050.150.149——100 2200.090.149——
In order to prevent such image fogging, Document 5: Japanese Published Unexamined Patent Application 7-36322 (Tokukaihei 36322/1995, published on Feb. 7, 1995) discloses a technique in which the left-over toner on the photoreceptor is removed by using a fur brush so as to prevent the toner from adhering to the electrification roller.
Note that, in the technique, when a large amount of toner is deposited on the fur brush, electrification failure and exposure failure cause deterioration of the image quality. Then, in order to prevent such deterioration of the image quality, a toner removing roller is provided so as to touch the fur brush. The toner adhering to the fur brush is removed by means of the toner removing roller.
However, in the technique of Document 5, it is necessary to use the toner removing roller so as to remove the toner adhering to the fur brush.
Thus, a structure of the printing apparatus is complicate, which brings about such a problem that the manufacturing cost increases.
Further, in order to prevent the image fogging, another conventional apparatus is arranged so that: a cleaning blade is made into contact with the photoreceptor drum, which scratches and removes the left-over toner from the photoreceptor drum before the left-over toner adheres to the electrification roller.
However, when the left-over toner is scratched and removed by such a blade, a slight amount of toner remains. Thus, this results in such a condition that: the more sheets are printed, the more left-over toner is deposited on the electrification roller.
In order to solve the fogging caused by the left-over toner deposited on the electrification roller, for example, Document 6: Japanese Published Unexamined Patent Application 10-254224 (Tokukaihei 254224/1998, published on Sep. 25, 1998) discloses a technique in which there is provided an air blow generating apparatus for generating air blow in the vicinity of a narrowest gap between the electrification roller (non-contact type) and the photoreceptor drum.
Further, in the technique, when the electrification roller is tainted with the left-over toner, the left-over toner on the electrification roller is removed by air blow so as to be returned to the photoreceptor drum.
Further, Document 7: Japanese Published Unexamined Patent Application 2001-209239 (Tokukai 209239/2001, published on Aug. 3, 2001) discloses a technique in which a cleaning member for electrically attract the toner is made into contact with the electrification roller (contact type) so as to remove the left-over toner on the electrification roller.
However, in the technique, it is necessary to provide the air blow generating apparatus, which brings about such a problem that the manufacturing cost increases.
Further, the technique of Document 7 is arranged so that the electrification roller rotates in a direction opposite to a rotation direction of the photoreceptor drum (with rotation). Thus, this arrangement brings about such a problem that the foreign material tends to be involved between the electrification roller and the photoreceptor drum.